Photo: The Canadian Press. All rights reserved.

A Free Syrian Army fighter walks past destroyed shops in Aleppo, Syria. Syrian rebels brought their fight within a mile of the heart of Damascus on Friday, seizing army checkpoints and cutting a key highway with a row of burning tires as they pressed their campaign for the heavily guarded capital, considered the likely endgame in the nearly 2-year-old civil war. (AP Photo/Abdullah al-Yassin)

Syrian troops backed by warplanes battled rebels for control of a key highway in Damascus Saturday, a day after opposition forces cut the strategic artery as part of what they say are efforts to lay the groundwork for an eventual assault on the heavily defended capital.

Rebels have been on the offensive in Damascus since launching a series of attacks on government positions on Wednesday. They brought their fight to within a mile of the heart of the capital on Friday, seizing army checkpoints and cutting a key highway as they pressed their campaign for the city, the seat of President Bashar Assad's power.

The fighting is the heaviest to hit Damascus since July, when a first rebel assault managed to capture several neighbourhoods before a punishing government counteroffensive. After that rebel foray, the regime quickly reasserted its control over the city, which has spared Damascus much of the violence and destruction that the civil war has wrought on other major urban centres.

Both the rebels and the government consider the fight for Damascus the most likely endgame in a civil war that has already killed more than 60,000 people. The city is heavily fortified and dotted with armed checkpoints, and activists say it is surrounded with three of the most loyal divisions of the army, including the Republican Guard and the feared 4th Division, commanded by Assad's brother Maher.

The latest Damascus offensive did not appear to be co-ordinated with rebels on other sides of the capital, and it was unclear whether the opposition fighters would be able to hold their ground.

Activists said the fighting on Saturday focused on a main highway that leads to northern Syria, a key road the regime uses to move troops and supplies. Rebels cut the road on Friday, and still controlled parts of it on Saturday despite government airstrikes and shelling to try to roll them back, said Damascus-based activist Maath al-Shami.

Activists say the fighting is the only beginning of a long battle for the capital.

"The attack was planned for more than 20 days and those responsible for it are army defectors," al-Shami said. "This is one of the stages to enter the capital. .... Storming Damascus is not easy."

He said only one checkpoint still stands in the way before the rebels reach Abbasid Square, a landmark plaza in central Damascus.